All excellent advice above. Weight loss is good, but not at the expense of strength loss. Building up the quad and glute muscles is critical. And training on uneven ground is important. Being able to maintain your balance in steep, rough terrain is what will keep you safe. If you fall and injure yourself, your hunt could be over. I start with a light pack of 25 lbs. or so and climb the steepist hills available, working my way up to 100 lbs. before the hunt. Carrying heavy loads over uneven ground will significantly stengthen the small-motor muscles around the knees and ankles that are essential to maintaining balance. If you don't have any hills nearby, stairs will work for stengthening your legs and butt, but will do little for your balance. And you must stenghten your downclimbing muscles. Stairclimbing machines can't replicate this exercise. When you can carry a 100 lb. pack up a steep slope for an hour without stopping, you may be ready.
Finally, toughen yourself mentally. As a famous writer once said, sheep hunting is designed to deprive a man of all of the comforts of civilization to which he has become accustomed. You must go into the hunt determined to stick with it through the end. On my first sheep hunt, which was far and away my toughest, though I was young and fit, I had not killed a ram by the last scheduled day of the hunt. I struck a deal with my outfitter to keep going, determined that I would go home only with a ram or with a broken leg, but not with only sore feet and a sad story. I killed a tremendous ram the next day.
Good luck!
HT